1. Field of the Invention
The present invention is directed to a device for cutting glass, for example for separating a drop or a portion from a plastic glass skein which is emerging from a feeder. The device includes two shear arms mounted for rotation in a common frame with each of the shear arms being provided with a knife at its extreme end and means for periodically rotating the arms to move in a pincher-like closing movement toward one another and away from one another so that the knives execute a cutting process. In particular, the invention is directed to the improvement in the means for periodically rotating the arms.
2. Prior Art
In a glass cutting process such as the process used to cut glass in a drop feeder, the knives move in a path during a working cycle which have a plurality of different individual segments. These segments are the segment as the knives are cutting the glass skein, the segment as the knives are being separated from the glass, the segment of the path of the knives moving to and from the glass skein as well as the segment of the path of the knives being held in the removed position. It is desirable to select the speed of movement of each of the knives in the individual path segments at an optimum speed or level for each of these path segments independent on the machine drive. By so doing, one achieves the shortest time for a working cycle without exceeding the speed in any critical path segment, for example the speed of the knives cutting the glass. It is also desirable to have a device in which a large range of cuts per unit time can be achieved. Thus, it is desirable to maintain the optimum speed for each of the segments of the working cycle without any change over the large range of cuts per unit time without requiring conversion of the device or replacement of the drive elements, for example a radial cam.
Known devices only partially fulfill the above mentioned desirable conditions. In a book by Giegerich and Trier, Glasmaschinen, Springer-Verlang, Berlin/Gottingen/Heidelberg, pp. 158, 159, a device for cutting glass is described which is driven by means of the cam plate which rotates at a uniform speed, which, however, is dependent on the number of cuts per unit time to be executed and thus the device directly depends on the machine drive. If a change in the number of cuts per time unit occurs, the speed of the temporal movement of the knives during each segment also changes which leads to changes in the cutting condition. In order to overcome this disadvantage one has a recourse to utilize a different cam for different ranges of the number of cuts per time unit and one must accept changes in the cutting conditions within the cutting count range of a given cam. For example, for a cutting count range of 5 to 110 cuts per minute, 20 different curves are required as shown by the chart D3 on page 174 of the above mentioned book. Also, a cut count of less than 3 per minute cannot be achieved or can only be achieved with the greatest difficulty. In this known cutting device, only the power for opening the shear is produced by the following roller of the cam. The force for closing is generally by means of a mechanical or pneumatic spring. This force is either permanently effective, for example, also opposing opening the knives and namely opposite the force of the cam follower lever or can be switched off upon opening, for example, in the case of a pneumatic spring. When the force is only applied during closing, a reversing process is required which, in turn, requires switching elements that are susceptible to wear and are also subject to frequent maintenance. Thus, additional outlay will occur with this device.
Another known drive for a device for cutting glass is disclosed in U.S. Pat. No. 2,812,619. In this device, the drive utilizes a hydraulic or pneumatic piston in a cylinder. With this drive, it is indeed possible to let the movement of the knives proceed with a constant speed over a relatively large range of cut counts per time unit. However, the device does not allow the speed sequence to be optimally adapted to the individual movement phases of the knives. For reversing the respective direction of movement of each knife, a reversing process of the piston is required and mechanical stops are usually required for the exact positioning of the knives, particularly the final position for closing the knives in the cutting operation. The reversing process for the piston requires a length of time, which is independent of the respective cut count and thus, must be readjusted upon changing the number of cuts per time unit if a corrected cut and a short glass contacting time are to be achieved. Interruptions in the course of manufacture arise because of this readjustment. On the other hand, the knives are caused to vibrate due to the reciprocal motion of the piston and the striking of the shear arms on the mechanical stops used to hold them in the desired position. These vibrations will cause the quality of the cut to be negatively influenced. The momentum required for perpendicular descent or for the displacement of the drop of glass fluctuates and the drop of glass will fall irregularly into the form or mold. This irregular falling of the drop will have a negative effect both on the course of the production and the quality of the production.